Foundations on clay soil

Blog Post
Digger digging into foundations on clay soil

Take extra care when excavating foundations in clay soils, especially if there are trees nearby. Clay soils are made up of 40% water, but trees can change this amount differently throughout the year - causing the soil to shrink or swell with enough force to affect the foundations of a building and so it's vital that the foundation is the right depth to prevent movement.

You can tell if your soil is clay by giving it a squeeze - if it stays in the shape you've made with it and doesn't crumble, it's probably clay (but in very dry conditions even clay will crumble).

Plastic clay

There are three types of clay in the UK, classified by their 'plasticity' (or how much they can change volume because of their water content):

  • Clays with the highest plasticity (and so the highest risk) are generally found in the South East of England, stretching up through the East Midlands to the Humber in the North and down to Bath in the West.
  • Medium plasticity clays are found in the rest of the South East, across the Midlands and up beyond the Humber Estuary towards the North East. There are also some isolated areas in the North West of England near the coast.
  • The rest of England and Wales are generally low plasticity clays - but they still carry some risk.

Minimum depths

Seasonal changes affect clay soils - causing them to swell in winter and shrink in summer. That's why there are minimum foundation depths for each type of clay. Strip, trench fill or pad foundations must be cast at a minimum of 750mm in low plasticity clays, 900mm in medium, and 1000mm in the highest risk areas.

Use the free Foundation Depth Calculator tool

The trouble with trees

But these minimum depths are not enough where there are trees. In drought trees will suck moisture from the clay, shrinking the soil and causing serious settlement and cracking in properties with shallow foundations. Cutting a nearby tree down won't help either.

In normal weather conditions clay adapts to the way nearby trees use water and maintains a steady amount - with the trees drawing out what they need. If you remove a mature tree, the clay won't be able to contain the excess water and will swell - sometimes with enough force to move foundations and lift concrete ground floors, causing serious structural damage.

So cut down the tree or leave it, you're snookered either way! The only answer is to form the foundation below the area affected by the trees.

Not sure of your soil type? Visit http://mapapps2.bgs.ac.uk/ukso/home.html

Dig deep

A little known fact about building control surveyors in areas with shrinkable clay is that they're all tree experts - or at least they often have a 'book of trees' in the car. That's because the type of tree determines the depth of foundation - they all have different water demands. The worst culprits are very tall trees, such as Poplars and other large broad-leafed species.

If you have a 'high water demand' tree the effect on foundation depth can be huge - often needing trenches down to 3m deep, with additional precautions such as compressible boards for heave protection. So how can you determine the depth required?

The Foundation Depth Calculator

Use the freely available LABC Warranty Foundation Depth Calculator, which covers the vast majority of trees found in the UK. All you have to do is enter the type of soil, type of tree, and the distance from the building and this online tool will show you the depth of foundation you need.

This is very handy when pricing a job as you can give a much better idea of the foundation costs to your customer beforehand. However, foundations should always be designed by a competent person prior to commencing work.

View the LABC Warranty Foundations section in their Technical Manual

Read our other articles about working with foundations


Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.

This article was reviewed and updated on 10 August 2023

Comments

(No subject)

Submitted 5 years 6 months ago

Very helpful and easy access information. Thank you LABC

(No subject)

Submitted 5 years 6 months ago

I’m doing my course work and I just want to double check the depth of my pile foundation. The ground condition: pockets of soft clay up to 1.5m, mature sycamore tree with TPO 12m the distance from crown point of the building line and sulphates ave. depth of 800mm to 1.5m.

(No subject)

Submitted 5 years 6 months ago

What do you think of waffle pod for very swelling soil

Webmaster note

Submitted 5 years 6 months ago

All comments posted at an earlier date to this one have been transferred from our old website.

Excavation

Submitted 5 years 5 months ago



How do varying ground conditions affect the depth of my dig could u Give some examples of different conditions and the effects they would have on the job.
Thanks

Foundation depth Calculator.

Submitted 4 years 12 months ago

This is a very misleading calculator appears to be devised by an insurance company. The is no guidance on its suitability, limits of application and classification of soil. The geographical (Post Code) identification is imprecise & totally inappropriate. This is at odds with other professional guidance. In the hands of an inexperienced BCO it has a potentials to give LABC a bad name.

Reply to: Foundation depth calculator

Submitted 4 years 12 months ago

Hi, thanks for your feedback.

There are terms and conditions to specifically clarify how the calculator may be used. We've updated the links to the calculator – users are required to accept the terms and conditions before use.

The foundation depth calculator should be used where LABC & LABC warranty is involved as our claims experience has proven that has been robust in its application.

The calculator was originally developed by a leading arboriculturist and does allow the user to select the plasticity of the soil if known.

As always, we welcome comments on the foundation depth calculator and we are currently in the process of updating it to include more tree types.

Regards,

John, LABC Warranty

Foundation engineering

Submitted 4 years 8 months ago

Nice piece. Very informative.

Site Analysis/ Construction Advice.

Submitted 4 years 6 months ago

I just got a site for a residential development, the site is characterized by deep pockets of soft clay to a depth of up to 1.5 metres in some areas. There is also evidence of sulfates to an average depth of up to 800 mm and in certain parts of the site, this goes to 1.5m. Also as indicated there are fully matured sycamore trees with TPOs order.
Please kindly advise me on suitable type or types of foundation and how effective to handle the present of sulphate on site.

Reply to: Site analysis/construction advice

Submitted 4 years 6 months ago

Hi, many thanks for your enquiry.

You'll need to engage the services of an engineer or a surveyor to design the foundations for you. This will need to take account of the proximity of the mature tree and the effect of the presence of sulphates. Once prepared you can submit your design for the foundations to your local building control team who'll be pleased to provide pre-application advice. You can find your local team by entering the postcode of your development in the 'find your council' bar at the top of our website.

Regards,

John, LABC

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